private static Size NewSize(int maxWidth, int maxHeight, int width, int height)
  {
   double w = 0.0;
   double h = 0.0;
   double sw = Convert.ToDouble(width);
   double sh = Convert.ToDouble(height);
   double mw = Convert.ToDouble(maxWidth);
   double mh = Convert.ToDouble(maxHeight);

   if ( sw < mw && sh < mh )
   {
    w = sw;
    h = sh;
   }
   else if ( (sw/sh) > (mw/mh) )
   {
    w = maxWidth;
    h = (w * sh)/sw;
   }
   else
   {
    h = maxHeight;
    w = (h * sw)/sh;
   }

   return new Size(Convert.ToInt32(w), Convert.ToInt32(h));
  }

  public static void SendSmallImage(string fileName, string newFile, int maxHeight, int maxWidth)
  {
   System.Drawing.Image img = System.Drawing.Image.FromFile(fileName);
   System.Drawing.Imaging.ImageFormat thisFormat = img.RawFormat;

   Size newSize = NewSize(maxWidth, maxHeight, img.Width, img.Height);
   Bitmap outBmp = new Bitmap(newSize.Width, newSize.Height);
   Graphics g = Graphics.FromImage(outBmp);

   // 设置画布的描绘质量
   g.CompositingQuality = CompositingQuality.HighQuality;
   g.SmoothingMode = SmoothingMode.HighQuality;
   g.InterpolationMode = InterpolationMode.HighQualityBicubic;

   g.DrawImage(img, new Rectangle(0, 0, newSize.Width, newSize.Height),
    0, 0, img.Width, img.Height, GraphicsUnit.Pixel);
   g.Dispose();

   // 以下代码为保存图片时，设置压缩质量
   EncoderParameters encoderParams = new EncoderParameters();
   long[] quality = new long[1];
   quality[0] = 100;

   EncoderParameter encoderParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
   encoderParams.Param[0] = encoderParam;

   //获得包含有关内置图像编码解码器的信息的ImageCodecInfo 对象。
   ImageCodecInfo[] arrayICI = ImageCodecInfo.GetImageEncoders();
   ImageCodecInfo jpegICI = null;
   for (int x = 0; x < arrayICI.Length; x++)
   {
    if (arrayICI[x].FormatDescription.Equals("JPEG"))
    {
     jpegICI = arrayICI[x];//设置JPEG编码
     break;
    }
   }

   if (jpegICI != null)
   {
    outBmp.Save(newFile, jpegICI, encoderParams);
   }
   else
   {
    outBmp.Save(newFile, thisFormat);
   }
           
   img.Dispose();
   outBmp.Dispose();
  }